Transition from open-pit to underground as a new optimization challenge in mining engineering

There are many deposits that have the potential to be mined by a combined method of open-pit and underground. In this manner, the most sensitive problem is the determination of the optimal transition depth from open-pit to underground or vice versa. To calculate this depth, a model based on block economic values of open-pit and underground methods together with the Net Present Value (NPV) attained through mining is first presented. During the model, NPV of open-pit is compared to the value of underground for the similar levels. A hypothetical example is used in order to analyze the model in detail. Based on the assumptions made such as: a discount rate of 15 %, each pair of contiguous level-cuts have to mine during one year, and one level as the height of crown pillar, the optimal transition depth was determined to be equal to 62.5 m. Then, level 6 was considered as the suitable crown pillar. Finally, maximum total NPV of the combined mining was calculated to be 25.54 units of currency.